There has long been an identified need for a liquid absorbent for ammonia vapor which overcomes the limitations of the currently used absorbent--H.sub.2 O--or more properly, aqua ammonia, i.e., a liquid solution of NH.sub.3 and H.sub.2 O including equilibrium amounts of NH.sub.4 OH, which is the actual composition which alternately absorbs and desorbs ammonia vapor in conventional ammonia absorption cycles.
The limitations of H.sub.2 O as absorbent for ammonia vapor stem from the appreciable vapor pressure of H.sub.2 O. The ammonia vapor which is desorbed from aqua-ammonia always includes a small fraction of H.sub.2 O vapor. The latter, if not removed by rectification, is detrimental in the subsequent evaporation step, as the NH.sub.3 preferentially evaporates, and a higher-boiling H.sub.2 O-containing liquid accumulates. The amount of H.sub.2 O in the desorbed ammonia vapor is determined by the difference in temperature between the generator, where the desorption occurs, and the condenser, which sets the desorption pressure. For temperature differences or "drops" of up to about 50.degree. C., the H.sub.2 O content is very small (on the order of 1 or 2%), and the amount of condensed NH.sub.3 which must be wastefully returned to reflux the rectifier rather than sent on to the evaporator is correspondingly very small. However, for higher drops the H.sub.2 O content and resulting reflux requirement rapidly increase to the point of unacceptability. This causes the cycle coefficient of performance (COP) to rapidly decline, e.g., from 0.8 at low drops to 0.3 or lower at very high drops.
The generator-condenser drop is closely related to and indeed the primary determinant of the evaporator-absorber "lift", i.e., the difference in temperature between evaporator and absorber.
Generically there are at least two absorption cycle applications which require high lifts (higher than those practical with NH.sub.3 --H.sub.2 O AWP (absorption working pair)) from low temperatures (well below H.sub.2 O freezing temperature). They are: industrial refrigeration, wherein evaporator temperatures below about -30.degree. C. are designed, with ambient-cooled absorbers at +30.degree. C. or higher (lift of 60.degree. C. or more); and space heating in cold climates, wherein ambient heat sources as low as -18.degree. C. are to be used, and heat pump discharge temperatures above 42.degree. C. are desired--once again a 60.degree. C. or higher lift. Whereas the NH.sub.3 --H.sub.2 O (aqua ammonia) absorption working pair is technically capable of satisfying the above applications, the low COP resulting from the water volatility at those conditions has precluded any known commercially viable application of ammonia absorption cycles at those high lift conditions.
What is needed, and one objective of this invention, is a non-volatile liquid absorbent for ammonia vapor, capable of high lifts without crystallization, and preferably capable of achieving a refrigeration COP of better than 0.5 and corresponding heat pumping COP of better that 1.5 at lifts in excess of 60.degree. C. Also, the absorbent should be thermally stable up to at least 145.degree. C., to permit the "drops" necessary for that lift. In addition, the absorbed and desorbed ammonia vapor should preferably be anhydrous to allow use of commercially available condenser and evaporator components.
Previous efforts to identify non-volatile absorbents for ammonia vapor have revealed many candidates, but none have achieved the required high lift without first encountering crystallization.
Early examples of searches for non-volatile absorbents for ammonia vapor include U.S. Pat. No. 1,734,278 and the technical article "Liquid Ammonia as a Solvent. I. The Solubility of Inorganic Salts at 25.degree. C." by H. Hunt, appearing in the September 1932 issue (Volume 54) of the Journal of the American Chemical Society, p. 3509-3512, New York.
More recently much attention has focused on LiSCN or NaSCN as absorbent. In volume 84 No. 7 p. 1075 of the above journal (April 1962), G. C. Blytas and F. Daniels describe NaSCN--NH.sub.3 properties. U.S. Pat. Nos. 3,458,445 and 4,691,532 describe use of that system in an absorption heat pump for residential heating and/or cooling. The properties of the LiSCN--NH.sub.3 system are described in the journal article "Thermodynamic and Physical Properties of Ammonia-Lithium THiocyanate System" by R. A. Macriss et al., appearing in the April 1972 issue (Vol. 17 No. 4) of the Journal of Chemical and Engineering Data, Washington, D.C.
Unfortunately neither of the above AWP's satisfies the need identified above. The system incorporating LiSCN is too thermally unstable, actually undergoing some visible decomposition at room temperature. The NaSCN-containing system is stable to above 145.degree. C., but does not provide an adequate width of solubility field-- at an ammonia vapor pressure of 1 ATA, the solution crystallization temperature is approximately 36.degree. C. Since atmospheric pressure boiling temperature of NH.sub.3 is -33.degree. C., the "lift" at the point of crystallization is only 69.degree. C., which does not provide an adequate margin against crystallization. For reliable operation, and allowance for upsets, a 15.degree. C. margin to crystallization is typically required, and hence what is needed is a non-volatile liquid absorbent for ammonia which crystallizes at or above 42.degree. C. at a pNH.sub.3 of 1 ATA.
The KSCN-NH.sub.3 AWP has a lesser solubility field (achievable lift) than NaSCN-NH.sub.3. However, it also has a less negative deviation from Raoultian behavior. Accordingly it has been disclosed that a mixture of KCNS and NaCNS in molar ratio between 1 to 1 and 3 to 1 will provide a higher COP than is possible with only NaCNS as absorbent. It was further disclosed that other thiocyanates such as calcium or lithium could be additionally included, West German Patent No. 3131487.
The above disclosure, while improving COP, unfortunately does not solve the problem defined above. The lithium and calcium thiocyanates are not adequately thermally stable, when mixed with Na and K in the defined ratio. And the Na-K thiocyanate mixture over the defined preferred range has a markedly lesser solubility field (achievable lift) than pure NaSCN.